PLEASE NOTE:


*

CCNet, 21/2000 - 15 February 2000
---------------------------------

     QUOTE OF THE DAY

     "The images of Eros recently sent back to Earth by the Near
     spacecraft show a surface covered with craters, some of which
     are quite large and degraded relative to Eros's size. This might
     seem surprising, given the general belief that Near-Earth
     Asteroids are fragments of main belt bodies that reached the
     terrestrial planet region via injection into powerful resonances
     like the 3:1 mean-motion resonance or the nu6 secular resonance.
     Since the median lifetime of NEAs is of order 10~My, NEAs are
     generally expected to have sparsely cratered surfaces. However,
     it appears that Eros may have spent a long time collisionally
     coupled to the main belt after its collisional birth. [...]
     We favor a scenario where many main belt asteroids slowly drift
     into resonance from neighbouring regions; this radial drift is
     probably caused by "Yarkovsky" thermal drag forces which act
     over the collisonal lifetime of the bodies [...]. In light of
     these new model results, we believe that many or perhaps most
     NEAs should have an age comparable to their collisional
     lifetime, presumably explaining the densely cratered surface of
     Eros."
         -- Alessandro Morbidelli and Bill Bottke, 15 February 2000


(1) LARGE NUMBER OF CRATERS ON EROS CONFIRMS NEW
    SCENARIO FOR THE ORIGIN OF NEAR-EARTH ASTEROIDS
    Alessandro Morbidelli and Bill Bottke

(2) NEAR'S HISTORIC FIRST IMAGE FROM EROS ORBIT
    http://near.jhuapl.edu/iod/20000214g/index.html

(3) FIRST LIGHT FROM EROS ORBIT
    Space Science News <express@spacescience.com>

(4) THE ICE MICROBES COMETH
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(5) BACKGROUND RISK FOR ASTEROID STRIKES
    Alan Boyle <alan.boyle@MSNBC.COM>

(6) CONFUSION ON "BACKGROUND RATE"
    David Morrison <dmorrison@arc.nasa.gov>

(7) PROBABILITY PROBLEMS
    Konrad Ebisch <kebisch@zycor.lgc.com>

(8) IMPACT PROBABILITIES
    Colin Keay <phcslk@cc.newcastle.edu.au>

(9) THE UNCERTAINTIES OF IMPACT PROBABILITY ESTIMATES
    Benny J Peiser <b.j.peiser@livjm.ac.uk>


================
CCNet-NEWS, 15 February 2000

(1) LARGE NUMBER OF CRATERS ON EROS CONFIRMS NEW
SCENARIO FOR THE ORIGIN OF NEAR-EARTH ASTEROIDS

By Alessandro Morbidelli and Bill Bottke

The images of Eros recently sent back to Earth by the Near spacecraft
show a surface covered with craters, some of which are quite large
and degraded relative to Eros's size. This might seem surprising,
given the general belief that Near-Earth Asteroids are fragments of
main belt bodies that reached the terrestrial planet region via
injection into powerful resonances like the 3:1 mean-motion resonance
or the nu6 secular resonance. Since the median lifetime of NEAs is of
order 10~My (Gladman et al., Science, 277, 197-201, 1997) NEAs are
generally expected to have sparsely cratered surfaces. However, it
appears that Eros may have spent a long time collisionally coupled to
the main belt after its collisional birth. 

Using a model presented at the last DPS in Abano which can
quantitatively reproduce the observed orbital and size distribution
of NEAs, we believe we can explain this inconsistency.  Our
computations show that 25-50% of the largest NEAs arrive on
terrestial planet-crossing orbits only after a slow increase of their
orbital eccentricity (discussed in Migliorini et al., Science, 281,
2022--2024, 1998;  Morbidelli and D. Nesvorny, Icarus, 139, 295-308,
1999).  The rest of the NEA population should reach planet-crossing
orbits through a "classical" fast track resonance (e.g., 3:1 or nu6
resonance). The number of bodies that are required to pass through
the fast- and slow-track resonances per million year, however, is
inconsistent with a dominant role of collisions in the
resonance-feeding process. For this reason, we favor a scenario where
many main belt asteroids slowly drift into resonance from
neighbouring regions; this radial drift is probably caused by
"Yarkovsky" thermal drag forces which act over the collisonal
lifetime of the bodies (Farinella and Vokrouhlicky, Science, 283,
1507-1510, 1998; Bottke et al. 2000, Icarus, in press), though
alternative mechanisms are under study. In light of these new model
results, we believe that many or perhaps most NEAs should have an age
comparable to their collisional lifetime, presumably explaining the
densely cratered surface of Eros.

P.S. A paper on this topic by Bottke, Jedicke, Morbidelli, Gladman
and Petit is under revision for publication in Science

For further information, please contact

Alessandro Morbidelli <morby@obs-nice.fr>
Bill Bottke < bottke@astrosun.tn.cornell.edu>

==================
(2) NEAR'S HISTORIC FIRST IMAGE FROM EROS ORBIT

NEAR image of the day for 2000 Feb 14
http://near.jhuapl.edu/iod/20000214g/index.html

Today at 10:33 AM EST the NEAR spacecraft was successfully inserted
into orbit around 433 Eros, becoming the first artificial satellite
of an asteroid. Just over an hour later, NEAR pointed its camera at
the asteroid and took this picture from a range of 210 miles (330 km)
above the surface. Mission navigators and operators will use this
image and others to be taken later to traingulate on landmarks on the
asteroid's surface, precisely measuring position of the spacecraft to
plot NEAR's course.

Features as small as a 100 feet (30 meters) across can be seen. This
view shows the 3-mile (5-kilometer) impact crater which the spacecraft
has spied for over a week during its approach. The two smaller craters
superimposed on its rim are each about 1.2 miles (2 kilometers) across.
An enormous boulder a full 170 feet (50 meters) in size sits on the
large crater's floor. Other key features of the surface are shallow
subsurface layering exposed near the tops of crater walls, and
shallow grooves crossing the surface and cutting the crater's rim.

--------------------------------------------------------
Built and managed by The Johns Hopkins University Applied Physics
Laboratory, Laurel, Maryland, NEAR was the first spacecraft launched in
NASA's Discovery Program of low-cost, small-scale planetary missions.
See the NEAR web page at http://near.jhuapl.edu for more details.

===================
(3) FIRST LIGHT FROM EROS ORBIT

From Space Science News <express@spacescience.com>

Space Science News for February 14, 2000

First Light from Eros Orbit:  NEAR's first close-up pictures from Eros 
orbit have arrived at Earth. This story includes a beautiful image of a
large crater on the asteroid and highlights from this afternoon's NASA
press briefing.  FULL STORY at

http://www.spacescience.com/headlines/y2000/ast14feb_1a.htm

====================
(4) THE ICE MICROBES COMETH

From Andrew Yee <ayee@nova.astro.utoronto.ca>

From NATURE, Friday, 11 February 2000
http://helix.nature.com/nsu/000217/000217-1.html

The ice microbes cometh
By PHILIP BALL

At the end of last year, microbes were found under thousands of metres
of ice in Antarctica. The discovery not only stretched the habitable
regions of the Earth to new extremes but also lent hope to the idea
that life might eke out a precarious existence on other worlds. Now new
research shows where these microbes might come from, and how they might
survive the rigours of a life in ice.

The plucky bacteria, encased in ice many thousands of years old, were
first reported last December by a team of US scientists[1]. They were
found in the bottom 100 metres of a core of ice drilled 3590 metres
into the ice sheet at East Antarctica's Vostok Station.

Why are the bacteria in the lowest part of the ice, not closer to the
surface, where they might have been deposited on wind-borne dust? The
answer is supplied by results reported in Nature[2] by Martin Siegert
of the University of Bristol and co-workers. Hidden beneath the ice
sheet on which Vostok Station stands is a vast lake, called Lake
Vostok, discovered in the 1970s.

Lake Vostok, all of which is below several kilometres of solid ice, has
been mapped out using radar signals, which bounce back from the ice at
the top and bottom of the lake to reveal its buried profile. At 670
metres deep and covering 14,000 square kilometres, it is the largest
known sub-ice lake.

Siegert's group has analysed radar data from airborne measurements
revealing that ice is being lost from the base of the sheet in the
north and west of the lake. To the south, on the other hand, the ice
over the lake is about 150 metres thicker on average, owing to freezing
of the lake water.

This suggests that ice is melting over one part of the lake and being
reformed over another. This may induce circulation in the lake water,
just as the water in surface lakes circulates because of convection. It
may also release rocky debris and other ice-bound impurities into the
water, which, the researchers say, could provide nutrients for any
organisms living in the lake.

It was precisely because of the presence of the lake that the ice core
was drilled. The core was taken from a region where refrozen ice had
been accreted from the lake onto the bottom of the ice, and the
drilling stopped just 120 metres short of the top of the lake. This
meant that it penetrated about 100 metres into the 'accretion' ice, and
it was here that bacteria were found -- some still living after being
released from the ice core. This suggests that they may grow within the
lake itself.

But how, asks physicist P. Buford Price of the University of California
in the Proceedings of the National Academy of Sciences[3], could
bacteria go on living within ice several degrees below freezing point,
at pressures four hundred times greater than the air pressure at the
Earth's surface?

Price says that the accretion ice above Lake Vostok provides all three
of the ingredients essential to life: water, energy and carbon. Glacier
ice, he points out, is laced with a network of water-filled veins
between solid ice grains, in which salts accumulate, lowering the
freezing point and preventing the veins from icing up. Price estimates
that these veins could be several thousandths of a millimetre across in
the Vostok accretion ice -- wide enough to accommodate bacterial cells.

The liquid veins also concentrate dissolved acids, including organic
acids such as formic and acetic acid (the main component of vinegar).
Price argues that chemical reactions involving these acids, which have
been detected from the ice-core studies, could provide sufficient
energy and carbon to support the number of microbes found in the ice
cores.

Lake Vostok is the best terrestrial analogue of Jupiter's moon Europa.
Over the past few years, the Galileo spacecraft orbiting Jupiter has
sniffed out strong evidence that below the crust of ice covering
Europa's surface lurks an ocean of liquid water stretching from pole to
pole. This is the only known world in the solar system other than Earth
on which a large body of liquid water seem likely to exist (although it
is possible that Callisto, another of Jupiter's moons, might also have
a subsurface ocean). If life can exist in the ice above Lake Vostok,
thousands of metres below frozen Antarctica, who is to say that it might
not be found also below the ice fields of Europa?

[1] Jouzel, J., Petit, J.R., Souchez, R., Barkov, N.I., Lipenkov, V.Y.,
Raynaud, D., Stievenard, M., Vassiliev, N.I., Verbeke, V. & Vimeux, F.
More Than 200 Meters of Lake Ice Above Subglacial Lake Vostok,
Antarctica Science 286, 2138-2141 (1999).

[2] Siegert, M.J., Kwok, R., Mayer, C. & Hubbard, B. Water exchange
between the subglacial Lake Vostok and the overlying ice sheet Nature
403, 643 (2000).

[3] Price, P.B. A habitat for psychrophiles in deep Antarctic ice. PNAS
97, 1247-1251 (2000).

Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE

=============================
* LETTERS TO THE MODERATOR *
=============================

(5) BACKGROUND RISK FOR ASTEROID STRIKES

From Alan Boyle <alan.boyle@MSNBC.COM>

dear benny:

regarding the "background risk" for an asteroid strike, i have to say
with regret that the initial versions of the story omitted the time
frame for the nasa estimate of "background risk" ... once it was
called to my attention i quickly added a phrase to indicate that this
risk applied "in a given year" -- perhaps don yeomans, paul chodas,
ron baalke or someone else could pass along information that would
surely be of interest relating to how that estimate was developed (i
assume it's based on the theory that such collisions occur about
every million years, more or less, though i'm not sure catastrophic
"deep impacts" happen that frequently).

i'm glad to hear better estimates as to the size of bf19 ... at the
time i wrote the story, i was just aware of andrea's "much less than
1 km" figure.

the story that i wrote also mentioned an estimate for the probability
of a 300-meter-wide asteroid:

"University of Hawaii astronomer David Jewitt said last month that
there was a 1 percent chance that Earth would be struck by a
1,000-foot-wide (300-meter-wide) object sometime in the next century."

... i guess that can be extrapolated to a 300-meter-wide asteroid
hitting earth every 10,000 years or so (which again might sound high
to some people, maybe low to others).

the story was also linked to an earlier one that mentioned jewitt's
speculation about the effect of such an asteroid:

"Such an impact would deliver a withering 1,000-megaton explosion and
cause perhaps 100,000 deaths," he said. "If the impact occurred in or
near a densely populated region - the eastern seaboard of the United
States, for instance, or Western Europe or coastal Asia - the
fatalities could easily rise into the tens of millions."

i hope this helps shed more light on the questions you raised (or at
least provides more grist for the mill). if CCNet subscribers have
any questions about anything i've written, i'd love to hear from
them... the feedback would certainly help our coverage of these sorts
of issues.
      
best, alan boyle, msnbc
       
p.s.: i'll look forward to seeing you and other e-mail penpals at the
aaas conference in d.c.

===================
(6) CONFUSION ON "BACKGROUND RATE"

From David Morrison <dmorrison@arc.nasa.gov>

Colin:

I don't really see your problem with the NASA news release from Don
Yeomans.

(1) The background risk is quoted for a one-year timescale ("per
year"), which is the way we always do it. I think this should be
clear both from context and by general usage. I agree that for a
news release it would be better to say "per year" explicitly,
however, rather than depending on context.

(2) The background rate depends steeply on the size of the object, and
we don't know the size of 2000 BF19, except that it is smaller than 1
km.  Thus any background rate must be a lower limit, corresponding the
the 1 km upper limit on the diameter. It is correct to say that this
lower limit is greater than one in a million, and to state the result
as an inequality. Thus I believe Yeomans is correct, and unambiguous,
to say that the background rate is higher than the estimated
probability for BF19 (all odds expressed as probability of impact per
year).  This seems straightforward to me, and it is also the way the
Torino hazard scale is defined and used.

(3) If you want estimated background impact rates for 50 KT and other 
specific energies, please refer to the technical literature. Published
estimates go back to the work of Shoemaker nearly two decades ago. 
See, for example, the rates given by Chapman and Morrison in their 1994
Nature paper on the impact hazard (and references there-in).  These are
refereed papers, available through any library.  Incidentally, the
surface impact rate for 50 KT is very near zero, since these objects
detonate at high altitude.

David Morrison
14 Feb 00

+++++++++++++++++++++++++++++++++++++++++++

David Morrison, NASA Ames Research Center
Tel 650 604 5094; Fax 650 604 1165
david.morrison@arc.nasa.gov or dmorrison@mail.arc.nasa.gov
website: http://space.arc.nasa.gov
website: http://astrobiology.arc.nasa.gov
website: http://impact.arc.nasa.gov

=====================
(7) PROBABILITY PROBLEMS

From Konrad Ebisch <kebisch@zycor.lgc.com>

Dear Benny,

In the latest of your reports, Colin Keay pointed out that impact
probabilities without a timescale are worthless. 

Your reply included this: 

3) the actual impact probability for objects this size
   (somewhere in the region of 1:20,000-50,000).

Please, not another estimate without a timescale. 

Nor a size range. The chance of being hit with a kilometer-size rock
(1km +/- 100m) is quite different from the chance of being hit with a
kilometer-size rock (1km +/- 500m).  If it means exactly 1 kilometer
(1km +/- 0), then the probability is zero. 

Nor a mention of what bodies will be hit. I assume this is intended for
impacts only with the Earth, but in some contexts collisions with other
bodies may be important too. 

Nor what is meant by "objects this size".  All objects this size?  An
average probability for any object with perihelion less than 1.2 AU?  

There is another thing in the NASA statement that is
unclear: 

"... asteroid that has not yet been discovered."

Not yet discovered as of 2000 February 7? 2022 January 1? Or not yet
discovered when it reaches the Earth's atmosphere? 

But let's not be too hard on this Don Yeomans' NASA press release.  For
those familiar with the context, I think that it is fairly clear that
they mean the time span to be the year 2022. Yes, it should have
specified. 

A major problem of the English language (and others) is that it is
often difficult to be precise and specific, and more so when speaking
to an audience that is not intimately familiar with the context. 

Yes, we all need to be more careful, but let's not condemn each other
for things that are sometimes unavoidable. 

Konrad Ebisch

===============
(8) IMPACT PROBABILITIES

From Colin Keay <phcslk@cc.newcastle.edu.au>

Dear Benny:

I thank my friends who responded to the point I raised. In the absence
of a time period for an estimate, "a year" is what I would have assumed,
as David Morrison points out. But members of the general public
interested in matters of potential collisions would interpret it
otherwise, usually taking a "lifetime" as the applicable period, i.e.
almost two orders of magnitude different. This is reinforced by
frequent statements comparing the chance of a person dying in
a plane crash with perishing due to an NEO impact.

I will follow up David Morrison's suggestion that I should consult his
and Clark Chapman's 1994 NATURE paper, although I feel the numbers will
have changed a little since then, and certainly changed since the
classic Shoemaker rates.

My suggesting 50 kT as an energy for which I'd like a probability
estimate was from the standpoint of ground destruction - large enough
to wipe out a city but small enough for the probility to be
significantly higher than for a 1 MT blast. Whether or not a colliding
object yielding 50 kT dumps it high in the atmosphere or closer to
ground zero must surely depend on its composition, a factor which could
be taken into account in the probability estimate using density
distribution data such as Ceplecha's.

If my query leads to a reduction in the number of woolly risk estimates
reaching the general public it will have been well worth while. After
all, scientists should not leave it to the scary news stories or
block-buster movies to convey essential warnings to the world.

Cheers ..... Colin Keay, Physics Department, University of Newcastle,
NSW, Australia (whose government couldn't care less).


Dr Colin Keay      :::::::     ~      ~   To  achieve  anything  really *
Physics Dept     ~       :::::      ~      worthwhile in research it is *
Newcastle Univ        ~       :::\ | /   ~  necessary to go against the *
NSW, AUSTRALIA 2308 ~      ~     - o -       opinions of one's fellows. *
phcslk@cc.Newcastle.edu.au       / | \  ~        "Where the Wind Blows" *
www2.hunterlink.net.au/~ddcsk        ~       ~       ~    - Fred Hoyle  *

===============
(9) THE UNCERTAINTIES OF IMPACT PROBABILITY ESTIMATES

From Benny J Peiser <b.j.peiser@livjm.ac.uk>

I should perhaps clarify my comment in yesterday's CCNet: Our
knowledge of the size of 2000 BF19 has not really improved. In fact,
the size estimates of asteroid 2000 BF19 are still pure guesswork. I
understand that there are no colour observations of the object. The
absolute magnitude of this asteroid has been revised from 19.6 to
19.2 and it seems likely that 2000 BF19 might be smaller than 1 km
across. But given the possibility that the magnitude may still
change, in addition to the fact that we have no idea what the albedo
is, even this vague size estimate may still have to be revised
significantly.

If we take all of these limitations into account, it is obvious that
impact probability calculations for 2000 BF19 still are rather vague.
I, for one, would like to know, for instance, exactly how Andrea
Milani calculated his 1 a million chance of impact for an unknown
date in 2022.

The problems with the *annual* background impact risk for similar
objects of an unknown size between, say, 500-1000 m are actually even
more problematic: they vary from somewhere between 1:20,000 -
1:1,000,000! It is this level of inherent uncertainty we have to live
with for the time being. That's why it is so important to eliminate
"virtual impactors" - whenever possible.

Benny J Peiser

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